High-frequency modulating system



March 13, 1928.

H. C. SILENT HIGH FREQUENCY MODULATING SYSTEM Filed May 24, 1924 uvwszvroR ATTORNEY Patented Mar. 13, 1928.

UNITED STATES 1,662,081 PATENT OFFICE.

HAROLD C. SILENT, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A'GORPOR-ATION OF NEW YORK.

HIGH-FREQUENCY MODULATING SYSTEM.

Application filed May 24, 1924. Serial No. 715,701."

This invention relates broadly to systems for modulating high frequency current and particularly to means for coupling the modulating device with the source of high frequency current to be modulated.

In the patent to Heising 1,442,147, which issued on January 16, 1923, there is disclosed a system of modulation in which the modulation isetfected by variably diverting portions of the normal space current of a vacuum tube oscillator, that is to say, the modulating current varies the effective electromotive force of the source of a space current of the oscillator so that the output current of the said oscillator will be modulated by and in accordance with the signals which produce the modulating current. In the arrangement disclosed by Heising for carrying out his method of modulation the plate electrodes of the modulating tube and of the oscillating tube are connected with a common source of plate potential, the said connection including a choke coil or auto-transformer which establishes a unity ratio coupling be tween the modulating and the oscillating tubes. The manner in which this circuit functions to effect the desired result is fully described in the said patent and is also well known to those, skilled in the radio art and V consequently it need not be described herein. Experience has shown that in order to get the maximum power from such circuit the impedance of the plate circuit of the modulator should equal that of the plate circuit of the oscillator.

It is the object of this invention to provide a method and means by which this equality of impedances may be readily obtained. I

This invention will be clearly understood from the following description when read in connection with the attached drawing of which Figure 1 shows an embodiment of the invention in which a single modulating tube is employed; Fig. 2 is an embodiment of the invention in which the modulating circuit is of the push-pull type and the coupling is a multiple winding transformer; Fig. 2 shows a variation of a detail of Fig. 2, and Fig. 3 is a modification of the'arrangement shown in Fig. 2 in which a variable auto-transformer is substituted for the multiple winding transformer of Fi 2.

In Fig. 1 t e transmitter 1 is coupled to the grid circuit of the modulator 2 by means of the transformer 3. The plate 4 of this modulator is connected through the impedance 5 with the windingo' of the autotransformer with which is connected the positive pole of the plate battery 7. The negative pole of this battery is connected by conductors 8 and 9 with the filaments of the modulator 2 andthe oscillator '10, re

spectively. The plate 11 of thsoscillator 10 is efiectively connected with the autotransformer 6 the connection including the winding 12, the impedance l3 and the movable contact 14 of the auto-transformer. The grid of the oscillator10 is connected with the tuned circuit made up of the winding 15, and the condenser 16. Windings 12 and 15 are inductively related with the winding 17 of the transmitting antenna. It is desirable to make clear that the invention is not concerned with a particular kind of 4 and the filament of the modulator 2 and conductor 8; the other branch including the impedance 13, winding 12, the space between the plate 11 and the filament of the oscillator 10 and conductor 9. When variations are impressed upon the grid of the modulator 2 the plate current of the modulator will vary in accordance with the grid variations. This will cause a variation in the total current through the winding 6 of the auto-transformer which varies the potential difference across said winding. Consequently, the plate potential of the oscillator 10 will be varied accordingly and the output current of the oscillator will be modulated in accordance with the current variations applied by the transmitter l to the grid of the modulator 2.

It has been found that the maximum power can be derived from the oscillator by making the impedance of the plate circuit of the modulator 2 equal to the impedance lib.

contact point 14. of the auto-transformer, this equality of plate circuit impedance. maybe obtained.

The arrangement shown in F ig. 2 illustrates the useof a modulator of the pushpull type adapted to a circuit for carrying out theHeising method of modulation. As iswell known to those skilled in the radio and vacuum tube art the push-pull'type of circuit produccsless distortion than the nonsymmetrical type of vacuum tube circuit, and-permits of greater power output for the same number of tubes. The output side of this push-pull modulator which includes the plate circuits of the tubes 2 and2' is coupled y means of the multiple winding transformer 18 with the plate circuit of the oscillator 10. The source of plate voltage 7 is connected with one terminal of the winding 19, and the source 7 connected with the midpoint ofthe winding 20 of the same trans- .former. Current will therefore flow from the positive pole of the battery 7 through winding 19 to the plate of the oscillator 10 and from the battery 7' through the two halvesof the winding 20 to the plates of the modulator tubes 2 and 2. When current variations are produced by the transmitter l, the grids 21 and 22 will be at different potentials and consequently the current flowing from the source 7 through the two halvesof winding 20 will be different. The resultant current will produce a dili'erence of potential across winding 19 which will vary the plate potetial of the oscillator 10. By varying the ratio of the windings l9 and 20 the impedance of the platecircuit ofthe oscillator 10 may be made. equal to the impedanceof the push-pull modulator which condition insures maximum power output.

The arrangement shown in Fig. 2 employs a. single source of plate voltage to vsupply windings 19 and 20. This permits I the use of tubes of different types in the improved.

oscillator. and in the modulator. The descript-ion of the operation as given above would be equally applicable in this case.

The. arrangement shown in Fig. 3 illustrates. the use ofan auto-transformer in place of the multiple winding transformer 18 shown in Fig. 2.

By means. of this improvement upon the well known Heising circuit the eliiciency of operation of this circuit may bematerially While this invention has been disclosed as embodied in part cular forms, itis to be understood that it not so limited but is capable of embodiment in other and diii'erent forms without departing from the spirit and scope of the appended claims.

IVhat is claimed is:

1. In a modulating system in which the modulating current varies the effective E. M. F. of the source of space current of a vacuum tube oscillator, the combination with an oscillator of a modulator and means to couple the said oscillator and modulator, the said means being so adjusted that the impedance of the circuit looking toward the said modulator is made equal to that looking toward the said. oscillator.

2. In a modulating system in which the modulating current varies the effective E. M. F. of the source of space current of a vacuum tube oscillator, the combination with a vacuumtubc modulator of a vacuum tube oscillator, a transformer couplingthe plate circuit of the modulator with the plate circuit of the oscillator, sources of plate current effectively connected through the windings of said transformer with the plates of the i'nodulator and oscillator tubes, the said coupling transformer being adjusted so that the impedances of the plate circuit of the modulator and of the plate circuit of the oscillator are equalized.

, 3. In a modulating system in which the modulating current varies the effective E. M. F. of the source of space current of a vacuumtube oscillator, the combination with an oscillator of a push-pull modulator, and

.a coupling between the said oscillator and equals the impedance of the input of the said oscillator.

4. In a modulating system in which the modulating current varies the effective E. M. F- of the source of space current of a vacuum tube oscillator, the combination with an oscillator of a push-pull modulator and a coupling transformer having one winding connected across the plates of the modulator, and a second winding connected with the plate of the oscillator, and sources of direct current potential connected with the midpoint of the said first winding and with one of the end terminals of the said second winding, the impedances of the plate circuits of the modulator and the oscillator, as viewed from the coupling transforu'ier, being equal.

In testimony whereof, I have signed my name to this specification this 19th day of May 1924.

, HAROLD C. SILENT. 

